In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique to transfer, on a silicon substrate or the like, a fine pattern which is required for forming an integrated circuit comprising such a fine pattern. For miniaturization of semiconductor devices, it has been attempted to further refine the resolution limit, for example, by a combination of ArF laser (wavelength: 193 nm) and an immersion method, but such a conventional photolithography method has been close to the limit. Under the circumstances, as an exposure technique for further miniaturization, an EUV lithography is considered to be promising which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, “EUV light” means a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm ±0.3 nm.
EUV light is apt to be absorbed by any substances and the refractive indices of 13.5 nm±substances are close to 1 at this wavelength, whereby it is impossible to use a dioptric system like a conventional photolithography employing visible light or ultraviolet light. Therefore, in EUV photolithography, a catoptric system, i.e. a combination of a reflective photomask and a mirror, is employed.
A mask blank is a laminate before patterning, to be used for the production of a photomask. An EUV mask blank has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light are formed in this order on a substrate made of e.g. glass. As the reflective layer, a Mo/Si multilayer reflective film is usually employed wherein a molybdenum (Mo) film as a high refractive index layer and a silicon (Si) film as a low refractive index layer are alternately laminated to increase the light ray reflectance when the layer surface is irradiated with EUV light.
For the absorber layer, a material having a high EUV light absorption coefficient, specifically a material containing chromium (Cr) or tantalum (Ta) as the main component, is employed.
A protective layer is usually formed between the reflective layer and the absorber layer. Such a protective layer is one provided for the purpose of protecting the reflective layer so that the reflective layer will not receive a damage by the etching process carried out for the purpose of forming a pattern in the absorber layer. Accordingly, for the protective layer, it is preferred to employ a material not susceptible to an influence by the etching process. Further, the protective layer is required not to lower the reflectance of EUV ray, since the reflective layer of the mask blank is irradiated with EUV light in a state where the protective layer is formed. For these reasons, as the constituting material for the protective layer, Ru or a Ru compound (such as RuB, RuNb or RuZr) is considered to be preferred.
In the production of an EUV mask blank, a sputtering method is preferably employed for the formation of the Mo/Si multilayer reflective film, the protective layer and the absorber layer for such reasons that a uniform film thickness can easily be obtained, the takt time is short, the film thickness can easily be controlled, etc. Here, for the formation of the Mo film and the Si film constituting the Mo/Si multilayer reflective film, and the protective layer, an ion beam sputtering method is preferably used, and for the formation of the absorber layer, a magnetron sputtering method is preferably employed.
The sputtering method is a film-forming method wherein a sputtering target surface is bombered by charged particles to beat out sputtered particles from the target so that the sputtered particles are deposited on a substrate disposed to face the target thereby to form a thin film. For a sputtering target to be used for such a film forming method, it is common to apply a structure wherein a target main body made of a film-forming material is held by a substrate so-called a backing plate.
Such a sputtering method has been used also at the time of forming wiring films, electrodes, element-constituting films, etc. of various electronic components in the production of electronic components such as semiconductor elements or liquid crystal display elements. In a film-forming step by a sputtering method in the production of such electronic components, generation of dust attributable to a target has been recognized as a serious problem (JP-A-2002-69627, JP-A-2002-146523). Here, “dust” is a fine particle having a diameter of e.g. at least 0.2 μm, and if such a fine particle is included in the formed thin film, it will be a cause for e.g. short circuiting between wirings or wiring disconnection, thus leading to a deterioration of the yield in the production of electronic components such as semiconductor elements or liquid crystal display elements.
From the basic principles of the target to be used for a sputtering method, the target main body has an erosion region and a non-erosion region.
Sputtered particles include ones which reach the substrate, ones which scatter around and ones which return to the target main body side. Among particles which return to the target side, particles deposited on the non-erosion region will not basically be sputtered again, and they will accumulate as a redeposition material as the sputtering progresses. If such a redeposition material is peeled by some factor, it will be included as dust in the formed thin film.
JP-A-2002-69627 and JP-A-2002-146523 propose a sputtering target which has been made possible to substantially reduce generation of dust by effectively suppressing a peeling, falling or splashing phenomenon of the redeposition material from e.g. a non-erosion region of a target main body, as a sputtering target to be used for forming wiring films, electrodes, element-constituting films, etc. of various electronic components, and a sputtering apparatus employing such a sputtering target.
In the case of the sputtering target proposed in JP-A-2002-69627, it is disclosed that by applying etching treatment or polishing treatment as surface treatment to at least a part of the non-erosion region of a target main body, it is possible to densify or planarize the redeposition material and further to improve the adhesion strength to the base material, whereby it is possible to effectively suppress generation of dust by peeling or falling of the redeposition material even at the later stage of sputtering.
Whereas, in the case of the sputtering target proposed in JP-A-2002-146523, it is disclosed that by forming a film to prevent peeling of redeposition particles by a PVD method or a CVD method on at least a part of the non-erosion region of the target main body, it is possible to densify or planarize the redeposition material and further to improve the adhesion strength to the base material, whereby it is possible to effectively suppress generation of dust by peeling or falling of the redeposition material even at a later stage of sputtering.
Here, the sputtering targets proposed in JP-A-2002-69627 and JP-A-2002-146523 are those used for a magnetron sputtering method.